The present disclosure generally relates to an organic light emitting diode (OLED) light source, and more particularly, to an OLED light source having multiple stacked OLED devices wherein each OLED device is removable from an underlying non-active OLED device.
An organic light-emitting diode device, also called an OLED, commonly includes an anode, a cathode, and an organic electroluminescent unit sandwiched between the anode and the cathode. The organic electroluminescent unit generally includes a hole-transporting layer, a light-emitting layer, and an electron-transporting layer, among others. A power source is attached to the anode and cathode, OLEDs are attractive because of their low drive voltage, high efficiency, high luminance, wide viewing-angle, and capability for full color displays, general lighting, and for other applications.
When a proper voltage is applied, the anode injects holes and the cathode injects electrons into the organic electroluminescent unit. The injected holes and electrons each migrate toward the oppositely charged electrode. When an electron and hole localize on the same molecule, an “exciton” is produced, which is a localized electron-hole pair having an excited energy state. Light is emitted when the exciton relaxes via a photoemissive mechanism. In some cases, the exciton may be localized on an excimer or an exciplex. Non-radiative mechanisms, such as thermal relaxation, may also occur, but are generally considered undesirable.
OLEDs can emit different colors, such as red, green, blue, or white, depending on the emitting property of its light emitting layer. Efficient white light producing OLED devices are considered a low cost alternative for several applications such as paper-thin light sources, backlights in LCD displays, automotive dome lights, office lighting, and the like. White light producing OLED devices should be bright, efficient, and generally have Commission International d'Eclairage (CIE) chromaticity coordinates of about (0.33, 0.33). In any event, in accordance with this disclosure, white light is that light which is perceived by a user as having a white color.
One of the problems with conventional OLED devices is with regard to luminance efficiency and operating lifetime. Over time, the device is subject to decay and as a result luminance drops as a function of time. By way of example, many conventional OLED devices have an operating lifetime of about 5 years or less before replacement is required due to a decrease in luminance, i.e., brightness, relative to an initial value. This operational lifetime further decreases when the OLED is driven at still higher brightness, where the higher brightness would allows making the device smaller and thus cheaper for some applications. Moreover, when OLEDs are built for flexibility and/or for speed of fabrication on thin barrier-coated plastic substrates or when they are encapsulated with thin-film barriers, small holes in the barriers can lead to the growth of dark spots overtime. Because of this, the use of conventional OLED devices as a light source has not been considered as a practical alternative to other types of lighting systems, e.g., LEDs, incandescent, fluorescent, and the like. To address the problem of luminance efficiency and operating lifetime, a tandem OLED structure (sometimes called a stacked OLED or cascaded OLED) has been disclosed by Jones et al, in U.S. Pat. No. 6,337,492, Tanaka et al in U.S. Pat. No. 6,107,734, Kido et al. in JP Patent Publication 2003/045676A and in U.S. Patent Publication 2003/0189401 A1, and Liao et al. in U.S. Pat. No. 6,717,358 and U.S. Patent Application Publication 2003/0170491 A1. This tandem OLED is fabricated by stacking several individual OLED units vertically and driving the stack using a single power source, wherein each of the individual OLED units are simultaneously powered by the single power source. The advantage is that luminance efficiency, lifetime, or both are reported to have been increased. However, as expected, the tandem structure increases the driving voltage approximately in proportion to the number of OLED units stacked together.
A need exists for OLED light sources that have increased operating lifetimes.